Your question seems to assume that the atheists here are affirmatively asserting "there is no God", where few here actually make that assertion. I think that you'll find most, instead, "reject your assertion that God (gods) exist".
An assertion that is logically possible (i.e. "God exists") cannot be disproven. As I've said in the past, if science advances to the point that Little Timmy and Little Susie have a Mini Hadron Collider and an abiogenesis kit and create miniature universes and life at will in their backyard, that will still not prove that God does not exist. It would merely demonstrate that God was not necessary for the universe or life to exist.
However, with that said, it is clear that the Genesis Creation is just another creation myth, that is not at all supported by the reality that we live in.
So if you wouldn't mind, answer me this... how would you expect our world to be different if there were no god?
- Babies would no longer be protected from horrible birth defects?
- Mankind would no longer be protected from natural disasters?
- God would no longer grant prayer requests for limb regeneration and other medical prayers that can't be otherwise cured or naturally go into remission?
- We would find unnecessary cruelty such as the reproduction of ichneumonid wasps?
- We would find many inefficiencies in living organisms?
- The vast majority of species that ever lived would be incapable of survival and go extinct?